The western U.S. is facing yet another summer
of severe drought. Science provides some answers as to what causes
droughts - and some baffling questions.

by Patrick L Barry and Dr Tony
Phillips

People often greet
the first warm days of summer with eager anticipation for the sunny
weather to come. But for many people in the western U.S., the arrival
of warm weather this year is an harbinger of hard times ahead.

Drought has gripped
some parts of the West for as many as seven consecutive years, causing
one of the worst dry spells in decades. Soils are dry; reservoirs
are low. Farmers and golf course managers are vying for irrigation
water, residents face water rationing measures, and the politics
of water "seniority" rights is heating up between cities and between
states.

Hope for a reprieve
fades with the departing winter, because little precipitation typically
falls in the West during summer months. These regions depend on
winter storms to stock the mountains with snow, which melts in summer
and replenishes water supplies. The snow pack in April 2004, though,
was only 40% to 75% of normal

The ongoing drought,
which has affected 20% to 50% of the land area of the contiguous
United States, isn't as bad as, say, the Dust Bowl drought of the
1930s. Then 70% of the U.S. was dry. In the Great Plains, precious
topsoil blew away, agriculture collapsed, farmers literally lost
their land. "The dispossessed were drawn west," wrote John Steinbeck
in The Grapes of Wrath. "Families, tribes, dusted out,
tractored out. Car-loads, caravans, homeless and hungry. They streamed
over the mountains - restless as ants, scurrying to find work to
do - to lift, to push, to pull, to cut - anything, any burden to
bear, for food."

What causes such severe
droughts? Are they predictable? Scientists aren't sure, but they're
learning by studying the current dry spell. Earth-orbiting satellites,
which didn't exist during the Dust Bowl years, now provide crucial
data about winds, rain, soil moisture, and the state of the oceans.
Somewhere among those numbers lies the answer.

A key factor is the
temperature of water in the Pacific Ocean, says Bob Oglesby, a climate
dynamicist at NASA's Marshall Space Flight Center. Sea surface temperatures
(SSTs) in the Pacific alter the course of the jet stream as it flows
eastward over North America. This high-altitude "river" of fast-moving
air is like a conveyor belt for storms, so the path it takes across
the continent has a strong effect on where rain and snow will fall.
By steering the jet stream, the Pacific Ocean acts like a baton-wielding
orchestra conductor directing the symphony of weather patterns across
North America.

Image courtesy US National Park Service.

Lake Mead, an important water source in the West created
in the 1930s by the construction of Hoover Dam, is approaching
record low levels - hence the "bathtub ring" around the lake,
shown here.

For example, a strong
"El Niño" pattern of warm Pacific surface waters near the equator
will drive storms into California, while the opposite "La Niña"
pattern steers moisture-bearing storms further north to Washington
state and Canada. One causes drought, the other alleviates it. But
there must be more to the story: While a mild La Niña lurked in
the Pacific during the onset of the current drought - as would be
expected - a shift to a weak El Niño in 2003 did not reverse the
drought.

"It's a really active
area of research right now as people are trying to decipher exactly
what's causing what," Oglesby says. He and his colleagues at the
Global Hydrology and Climate Center are among those working to understand
what's going on. In particular, Oglesby is investigating how the
land and atmosphere interact with each other during a drought, focusing
on the roles that snow cover and soil moisture play.

Part of the difficultly
in understanding drought lies in the fact that weather involves
many feedback loops that complicate its behavior and defeat simple
cause-and-effect explanations. Soil moisture creates such a feedback
loop during dry weather. Oglesby explains:

Most of the western U.S. is suffering from some degree of
drought. The darkest color on this map represents the most
extreme category of drought in NOAA's classification scheme.

"Once you get into
a dry pattern and you start to dry the ground out, that reduction
in soil moisture can help to intensify and perpetuate the drought."
Normally, the evaporation of soil moisture consumes much of the
energy contained in the summer sunshine; without this moisture,
that energy heats the ground instead and raises temperatures even
further. Warmer temperatures create a high pressure system which,
in turn, blocks storms from coming into the area. Drought begets
drought.

Drought is a natural
part of cyclical weather patterns in North America, notes Oglesby.
Physical clues about ancient weather, such as tree rings and lake
sediment cores, show that dry spells such as the Dust Bowl and a
similar drought in the 1950s typically occur a few times per century.
The historical record also reveals a "mega-drought," longer and
more severe than any recent episodes, 500 or so years ago.

In modern times "there's
more to drought than simple lack of precipitation," adds Roger Pielke
Sr., a state climatologist for Colorado and a professor of atmospheric
sciences at Colorado State University. "You have to consider human
factors like the amount of water being drained from rivers for crop
irrigation and drinking water. In absolute terms, the ongoing dry
spell is not yet as severe as the Dust Bowl of the 1930s, but the
impacts have been relatively severe because the demands that people
place on the water supply are so much greater now than they were
back then."

This makes a complicated
situation even more complicated. Land-use and water-use by humans;
large-scale atmospheric circulation changes caused by ocean temperatures;
feedbacks between the land and atmosphere: they all play a role.
Climatologists can't yet put these factors together to predict what
will happen many years in advance. Next winter is mystery enough.
Will it bring much snow ... and relief? No one knows.

One thing seems sure,
though: With levels of moisture in the soil and snow on the mountains
both below average, people in the western U.S. are facing at least
one more long, dry summer.